The field of light-emitting diodes and in particular the field of the so-called “white LEDs” form the background to the present invention. Since light emitting semiconductor elements as a rule emit light at a specific wavelength, the production of a semiconductor element which directly provides white light has not yet been achieved. Instead, the semiconductor elements are configured such that they emit light of an individual wavelength, for example in the colours green, red or blue. To make the generation of white light possible nevertheless, it is known from the state of the art to convert the light of blue LEDs by means of colour conversion into a white light mixture. Thereby a phosphor, which is normally embedded in a matrix, is arranged around the semiconductor element—the so-called LED die. The blue light is now absorbed in the surroundings of the LED die by the phosphors and then converted into longer wavelength light. This longer wavelength light of the phosphors, in combination with the unconverted blue light of the light-emitting diode chip, then results in a white light mixture.
The forms for the arrangement of the colour conversion material around the LED die which are known from the state of the art usually have, however, the defect that that the light which emerges from the colour conversion material surrounding the LED chip is not homogeneously white but has different colour according to spatial direction and/or has different colouring at the surface of the phosphor layer. With collimation and imaging of the emitted light—for example with aid of a lens—this leads to reinforcement of the colour inhomogeneities.
The cause of these inhomogeneities lies in that the colour converted component increases proportionally to the path of the light through the phosphor/matrix surroundings. Due to the fact that the light-emitting diode chip, which is usually configured to be cubic or cuboid, imitates light in all directions it is very difficult to realize a uniform conversion of the light. A considerable disadvantage now, however, results from this for an imaging optical system which is to realize homogeneously white light from an inhomogeneous light emitting area. White LEDs therefore typically have, with conventional optical system, an inhomogeneous white emission the imaging of which is coloured bluish in the centre and yellowish at the margins.
Various arrangements are known from the state of the art with which it is attempted to convert the light emitted by a light-emitting diode chip as homogeneously as possible into a white light mixture. Two known configurations are illustrated in FIGS. 5 to 7 and are discussed briefly below.
The first known light-emitting diode arrangement 100 in FIGS. 5 and 6 has firstly a light-emitting diode chip 101 which is arranged on a base 102. The base 102 consists for example of a thermally conductive insulating layer 103, on the upper side of which there is located an electrically conductive layer with conductor paths 104, on which in turn the LED die 101 is arranged. The electrical contacting of the LED die is effected to the side from the conductor paths 104 by means of bonding wires 105 which lead to the upper side of the light-emitting diode chip 101.
To colour-convert the light emitted by the light-emitting diode chip 101, this is surrounded by a hemispherical shaped encapsulation which is filled completely with a colour conversion material 106. With this arrangement it is sought to attain a uniform surface colour through the approximately uniformly long path of the light rays through the hemispherical shaped encapsulation. The emergence of the light, with a refractive index transition of n≈1.4 to 1.6 for the encapsulation with respect to air with n=1, provides—corresponding to the illustration in FIG. 6a—local equalisation of the part emission of the non-converted blue light and of colour-converted light converted in colour by the phosphors of the colour conversion material 106. Through this the resulting light mixture is homogenized with regard to its colour.
The known construction requires, however, that the light-emitting diode chip 101 can essentially be considered to be a point and the encapsulation therefore must be substantially larger than the light emitting area at the light-emitting diode chip 101, though. In turn this brings with it the disadvantage that for imaging of such a system a very large lens is needed, since for effective light directing the luminescent surface should act approximately as a point also for the lens. The size of the mimumum usable hemisphere for colour conversion is correspondingly approximately three to four times the edge length of the light-emitting diode chip 101, whilst however the size of the lens should be approximately ten to twenty times the luminescent surface.
FIG. 6 shows a complete arrangement for a white light LED in which a spherical converter geometry and a lens 107 including the arrangement is put to use. Since for the imaging of the light only a restricted angular range of the emission can be put to use, in the case of a hemispherical converter heavy lateral light losses are to be expected, i.e. a certain proportion of the light cannot be imaged by the lens 107 due to the emission characteristics.
If, however, instead of a light conversion material configured in form of a hemisphere there is used a coating with a constant thickness, then there is provided the arrangement illustrated in FIG. 7.
Firstly, this second variant of a known light-emitting diode arrangement 120 consists again of a light-emitting diode chip 121 which is arranged on a base 122 consisting of an insulation layer 123 and a layer with conductor paths 124. As in the case of the first known arrangement, contacting is here effected also by means of a bonding wire 125 which is led to the upper side of the light-emitting diode chip 121.
The colour conversion material 126 is now arranged such that it covers the surface of the light-emitting diode chip 121 all over and uniformly with a constant thickness. The light rays exiting the colour conversion material 126 are then again collimated by a lens 127.
If the coating is chosen to be too thin, then the electrode structure of the LED chip 121 is still visible, what with imaging at a small emission angle (e.g. less than 10°) leads to an uneven imaging in the incident light field. The electrode structure also can lead to colour unevennesses if the layer of colour conversion material 126 is very thin. This is for example the case then if larger areas of the chip 121 do not luminesce.
If the layer has, in contrast, a greater layer thickness, there is the risk that the edge of the layer emits in another colour. However, at least then the electrode structure would no longer be visible and colour inhomogeneities due to larger, non-luminescent electrode structures could also be avoided.
With this known arrangement the light rays emitted by the side areas can therefore be only partly used for imaging. With increasing layer thickness more and more light is emitted through the side areas, through which the quota of utilizable light is further reduced. This known configuration therefore also has disadvantages with regard to the achievable homogeneous white light emission and the possibility of imaging this by means of an optical system.
There are many intermediate geometries between the known configurations illustrated in FIGS. 5 to 7 for light-emitting diode arrangements, which however are not realized since in substance only the structural height for the light-emitting diode arrangement illustrated in FIG. 6 has an acceptable size.
A possibility for generating white light without use of a colour conversion material is described in WO 02/50472 A1. This publication describes a light-emitting diode arrangement with which several light-emitting diode chips of different colours are arranged on the base side of a funnel-shaped reflector. Through the arrangement of the differently coloured light-emitting diode chips, as well as a special configuration of the reflector, it is achieved that the light of the light-emitting diode chips mixes in total to white light. However, this known construction is very complex in comparison with the solutions illustrated in FIGS. 5 to 7, in which respectively individual light-emitting diode chips can be used.